Microscopic Aspects in Dicationic Ionic Liquids through the Low-Frequency Spectra by Femtosecond Raman-Induced Kerr Effect Spectroscopy

The interionic vibrations in imidazolium-based dicationic ionic liquids (ILs) containing the bis(trifluoromethylsulfonyl)amide ([NTf2]−) counteranion were investigated using femtosecond optical-heterodyne-detected Raman-induced Kerr effect spectroscopy. The microscopic nature of the dicationic ILs ([C n (MIm)2][NTf2]2, where n = 6, 10, and 12; MIm = N-methylimidazolium) was compared with that of the corresponding monocationic ILs ([C n MIm][NTf2], where n = 3, 5, and 6) used as reference samples. Low-frequency Kerr spectra within the frequency range 0–200 cm–1 of the ILs revealed that (i) the spectral profile of the dicationic ILs as well as that of the corresponding monocationic ILs is bimodal; (ii) the difference in the spectral shapes of the dicationic and monocationic ILs is greater for [C6(MIm)2][NTf2]2 and [C3MIm][NTf2] than for the ILs with longer alkylene linker/alkyl groups, namely [C10(MIm)2][NTf2]2 and [C5MIm][NTf2], and [C12(MIm)2][NTf2]2 and [C6MIm][NTf2]; (iii) the small difference between the dicationic and monocationic ILs is confirmed by the relative intensity of the low-frequency component (ca. 17 cm–1) to the high-frequency component (ca. 70 cm–1); and (iv) the spectral profiles of the three dicationic ILs are not very different, but the line-shape of the low-frequency Kerr spectrum of 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide ([C3MIm][NTf2]) is significantly different from those of the other two monocationic ILs whose cations have a longer alkyl group. The distinguished line-shape of the low-frequency Kerr spectrum of [C3MIm][NTf2] from the other ILs can be accounted for by the homogeneous nature in the microstructure of the IL, but the other ILs indicate microsegregation structures due to the longer nonpolar alkylene linker or alkyl group in the cations.